June 20, 1964
COMMUNICATIONS TO THE EDITOR
hibited carbonyl absorption a t 1658 cm.-l and benzylic methylene absorption a t T 5.45 p.p.m. (half-width = 2 c.P.s.). The benzylic proton on the chlorine-bearing carbon atom absorbed in the aromatic region. Integration of the n.m.r. spectrum indicated that there are 16 protons in the aromatic region and 2 in the aliphatic region.
2515
The Question of a Benzene Cation Insertion Reaction. A Novel Intramolecular Electrophilic Substitution'
Sir: Three of the products obtained upon thermal decomposition of the diazonium ion I in acetic acid containing sulfuric acid have been shown to be benzaldehyde, Nbenzylbenzamide, and 1-phenyl-2-benzylphthalimidine (III).3 Similar products were obtained from the N,Ndimethyl analog of I.3 Evidence has been presented that the first two of these products arise from solvolytic cleavage of the cation I1 presumably formed by intramolecular abstraction of a hydride ion by the benzene cation which is produced by loss of nitrogen from I . 4
I
Addition of antimony pentachloride to an ethylene chloride solution of V converted most of V into the cation 11, having absorption a t 1748 cm.-' ( s ) (c=O) 8+
Ci" I
6+
and 1585 cm.-l (vs) (-CH-=N) in its infrared spectrum and a t T 0.48 (half-width = 7 c.P.s.) and 4.30 p.p.m. (half-width = 3 c.P.s.) in its n.m.r. spectrum12 (area ratio 1:2). The addition of chloride ion, in the form of trimethylamine hydrochloride, to the solution of I1 caused the regeneration of V. Addition of water caused the cleavage of 11, furnishing benzaldehyde and N-benzylbenzamide (111). The hydrolysis also produced small quantities of benzoic acid and benzylidine benzylamine (IV). The yield of the latter two products increased substantially when the hydrolysis was performed on a solution of I1 which had been heated a t 70' for 1.5 hr.13 T h e production of the same f o u r hydrolysis products
f r o m both the synthetic cation 11, and the diazonium i o n I after its thermal decomposition, provides powerful chemical evidence, in addition to the spectroscopic evidence already cited, for the existence of' the cation 11 a s an intermediate in the diazonium i o n decomposition reaction. Another interesting property of I1 is its conversion to N,N-dibenzylbenzamide by hydride ion abstraction from tribenzylamine.1 4 , l5 This reduction serves as further proof for the structure of 11. An unusual intramolecular electrophilic substitution reaction of the cation I1 and its mechanistic implications are discussed in the accompanying communication. l6 (12) A fairly good model for I1 is the hydrochloride of I V , the nonaromatic protons of which absorb at 7 0.52 and 4.94 p.p.m. (13) Upon heating, the hexachloroantimonate and hexa0uorophosphate salts of I1 apparently decompose partially to benzoyl halide and the imine (complexed with the Lewis acid). (14) H. Meerwein, V. Hederich. H. Morschel, and K. Wunderlich, A n n . . 656, 1 (1960). (15) The ion I1 failed to abstract a hydride ion from triphenylmethane under the same conditions. (16) T. Cohen and J. Lipowitz, J . A m . Chem. Soc., 8 6 , 2515 (1964).
DEPARTMENT OF CHEMISTRY UNIVERSITY OF PITTSBURGH PITTSBURGH, PENNSYLVANIA 15213
I1
THEODORE COHEN LIPOWITZ
JONATHAN
RECEIVED APRIL 6, 1964
I11 One possible mode of formation of I11 involves an intramolecular electrophilic substitution by 11. However, it is questionable whether a relatively stable cation of this type could execute an electrophilic substitution a t a rate comparable to that of its solvolytic c l e a ~ a g e . ~ , ~ Furthermore, one might expect the positive carbon atom of I1 to attack the benzyl ring more rapidly than it attacks the deactivated benzoyl ring. The preference for attack on the deactivated ring can, of course, be explained by 'invoking the argument2 that the ortho position of the benzoyl ring is constrained by a rwonanceimposed planarity of the system to a position closer to the positive carbon atom than that occupied by the ortho position of the benzyl ring. Nevertheless, i t is entirely possible that the ring closure to a phthalimidine only occurs because i t very closely follows or is concomitant with the transfer of the hydrogen. Precedents for the formation of rings by the attack of cationic carbon on saturated carbon atoms are An intriguing possibility is that this type of carbon-carbon bond formation is particularly favorable in the present system because of the (1) Supported by the National Science Foundation through Grant N S F G-23705. (2) T . Cohen, A. H. Dinwoodie, and L. D. McKeever, J . Org. Chem., 27, 3385 (1962). (3) 1'.Cohen, R . M. Moran. and G. Sowinski, i b i d . , 26, I (1961); A. H. Lewin, unpublished results. (4) T. Cohen and J. Lipowitz, J . A m . Chem. Soc., 8 6 , 2514 (1964). (5) The N,N-dimethyl analog of I gives substantial quantities of 2methylphthalimidine even in aqueous solution.3 (6) A somewhat similar electrophilic substitution is involved in the Pictet-Spengler reaction.' In this case strong activating groups are usually required on the benzene ring being substituted. (7) W. M. Whaley and T. R . Govindachari, Org. Reocfions, 6 , 151 (1951). (8) L. de Vries and S. Winstein, J . A m . Chem. Soc., 81, 5363 (1960); P. S.Skell and I. Starer, ibid., 84,3962 (1962).
2516
COMMUNICATIONS TO THE EDITOR
availability of the rather stable u-complex IV as an intermediate. This species, which could be formed by an “insertion”g of the benzene carbonium ion into the C-H bond, is of course the same intermediate that would be involved if I11 were formed from 11. I t is in fact conceivable that the cation I1 could also be formed from IV rather than by a more direct 1,5-hydride ion transfer. The possibility of such an “insertion” reaction takes cognizance of the similarity between benzene carbonium ions and carbenes. ll.15
II
0 IV
The independent preparation of the cation 114 now makes it possible to study its intramolecular electrophilic substitution reactions. In dry ethylene chloride, I1 has been found to be stable with respect to fivemember ring formation for 24 hr. a t room temperature. In the presence of the base tribenzylamine, the phthalimidine I11 is formed in detectable quantities (n,m,r.) a t the end of 10 min. a t the same temperature.” This remarkable finding probably indicates that the proton removal from the presumed intermediate IV is the ratedetermining step in this electrophilic substitution, In the absence of added base, phthalimidine 111 was produced in 670 yield a t the end of 1 week at room temperature and in 1470 yield when the solution was heated a t 70” for 1.5 hr. None of the l-phenyl-2benzoylisoindoline2 which would result by substitution into the benzyl ring could be detected by gas chromatography. Thus, it is evident that this electrophilic ring closure reaction is controlled exclusively by the stereoelectronically imposed proximity of the reacting centers rather than by the electron availability on the ring in which substitution occurs. The low rate of the intramolecular electrophilic substitution makes it appear doubtful that ring closure could compete with solvolytic cleavage of the cation 11. A direct study of this competition was performed by injecting 1 ml. (0.7 mmole) of an ethylene chloride solution of the hexachloroantimonate salt of I1 into the acetic-sulfuric acid reaction mixture (30 ml.) a t the temperature (steam bath) of the previous2 diazonium (9) This “insertion” reaction would require attack of t h e cationic carbon on the carbon-hydrogen bond in a direction perpendicular t o it. This would presumahly involve a transition state of the very same type as t h a t suggestedlo on experimental and theoretical grounds for hydride transfer reactions (10) hI. F. Hawthorne and E . S . Lewis, J . A m . Chem. Soc., 80, 4296 (1958). (11) Both have only two 0-bonds and are thus capable of forming two more Both are very unselective.lz T h e existence of both singlet and triplet carbenest# might also have a n analogy in this system since evidence has recently been put forth t h a t such cations may exist in triplet states.“ (12) E. S I.ewis, J . A m . Chem S o c . , 80, 1371 (19581, W .E. Doering, K G Buttery, K G Laughlin. and N. Chaudhuri. ibid., 78,3224 (1956). (13) G . Herzberg and J Shoosmith, N a t u r e , 18.3,1801 (1959). (14) R . W T a f t , J . A m . Chem. SOL.,83, 3350 (1961) (15) T h e benzene carbonium ion may or may not be associated with a molecule of nitrogen 16 (16) E S . Lewis and J. M .Insole, J . A m . Chem. Soc., 86,34 (1964). (17) Competing with phthalimidine formation in this case is a hydride transfer from trihenzylamine t o I1
Vol. S6
ion decomposition in this solvent and examining the hydrolysis product by gas chromatography for the phthalimidine 111. None was found. If I11 were indeed formed from the cation I1 in the diazonium ion decomposition, a simple calculation based on the previous data2indicates that about 9% of this cation should be converted to phthalimidine I11 under these conditions. Although this cannot be taken as rigorous proof that the phthalimidine is formed by direct attack of the benzene carbonium ion on the benzylic carbon atom (or on the C-H bond), i t is certainly suggestive of this type of mechanism.18 Experiments now under way should throw more light on this question. (18) Many of the products t h a t have been reported in diazonium ion decompositions can be explained, although not uniquely, by t h e “insertion” process. For example, this path is much more attractive than a radical combination mechanism for t h e production of m- and p-nitrophenylethanols in the photolytic decomposition of t h e corresponding diazonium ions in ethanol.lg The formation of Ruorenes, in one case quantitatively, upon is another example. Finally, deamination of 2-amino-2‘-alkylbiphenyls2a the reductions t h a t frequently occur when diazonium ions are heated in alcohol solutions could be brought about by insertion of the benzene carbonium ion into t h e carbinol C-H bond to form a 0-complex followed by loss of a proton and an aldehyde or ketone. (19) W. E. Lee, J. G . Calvert, and E . W. Malmberg, J . A m . Chem. SOL., 83, 1928 (1961). (20) L. Mascarelli and B. Longo, Gazz. chim. itai., 67,812 (1937); Chem. Absfr., 99, 4564 (1938); L. Mascarelli and A. Angeletti, Gazz. chim. itai., 68, 29 (1938); Chem. Abstr., 39, 4565 (1938); L. Mascarelli and B. Longo, Gazz. chim. itai., 88, 121 (1938); Chem. Abstr., 39, 6235 (1936).
DEPARTMENT OF CHEMISTRY UNIVERSITY OF PITTSBURGH PITTSBURGH, PENXSYLVANIA 15213
THEODORE COHEN LIPOWITZ
JOXATHAN
RECEIVED APRIL 6, 1964
The Isomerization of 1,3-Cyclooctadiene to 1,5-Cyclooctadiene via the Rhodium(1) n-Complex
Sir : Recent reports on the isomerization of cyclooctadiene by iron carbonyl’ and by potassium t-butoxide in dimethyl sulfoxide2 have demonstrated that 1,3-cyclooctadiene is thermodynamically the most stable of the isomeric cyclooctadienes. By each method, l,3-cyclooctadiene is isomerized quantitatively to 1,3-cyclooctadiene. We wish to report that RhC13.3Hz0,which forms T complexes preferentially with chelating 1,5 dienes, under conditions which isomerize other olefins, isomerizes 1,3-cyclooctadiene t o I , the dimer of rhodium(I) chloride-l,5-cyclooctadiene.5~~ Analytically pure 1,5-cyclooctadiene is obtained by treating I with aqueous KCN.
(1) J. E.Arnet and R . Pettit, J . A m . Chem. Soc., 83, 2954 (1961). (2) D. Devaprahhakara, C . G . Cardenas, and P. D. Gardner, ibid., 86, 1553 (1963) (3) E. W .Abel, M . A. Bennett, and G . Wilkinson, J . Chem. Soc., 3178 ( 1959). (4) R .E. Rinehart, H . P. Smith, H. S. Witt, and H . Romeyn, Jr., J . A m . Chem. Sac., 84,4145 (1962).
